Household ammonia (ammonium hydroxide) is a common cleaning agent containing nitrogen, a primary nutrient in most fertilizers. When used in the garden, it can act as a non-selective herbicide and kill weeds. However, this effect is primarily localized and temporary, functioning as a contact killer rather than a long-term systemic solution. While effective for small, isolated weeds, this method carries significant risk and is not recommended for widespread garden use.
The Mechanism of Ammonia as a Herbicide
Ammonia destroys plant tissue through a two-pronged chemical attack on the weed’s cellular structure. The highly concentrated solution acts as a rapid desiccant upon contact with the foliage, quickly drawing water out of the plant cells. This immediate, non-selective contact action causes cellular rupture and visible burning, damaging any green plant material it touches. Simultaneously, the plant absorbs the ammonium ion (\(\text{NH}_4^+\)) through the treated leaves.
Plants naturally convert ammonium into other compounds, but an overwhelming dose causes a buildup of toxic levels within the cells. The inability to process this excessive nitrogen disrupts internal metabolism, leading to chemical shock and cellular damage. Because the action is localized and does not travel effectively through the plant’s vascular system, it is most effective on small, annual weeds with shallow root systems.
Preparing and Applying Ammonia for Weed Control
To effectively kill weeds, the ammonia solution must be highly concentrated to ensure rapid desiccation and toxicity. A common recommendation is to use a 50/50 mixture of standard household ammonia (typically 5 to 10 percent ammonia by weight) and water. This concentration is significantly stronger than what is used for fertilization, ensuring a phytotoxic effect.
The best time for application is on a sunny, dry day with no wind, as heat enhances the burning effect and prevents drift onto desirable plants. The application must be highly targeted, focusing only on the foliage of the unwanted plant. Use a dedicated plastic spray bottle or a small garden sprayer to control the stream and minimize overspray. For the solution to work as a contact killer, thoroughly wet the entire leaf surface of the weed.
This method is generally ineffective against perennial weeds, such as dandelions or thistle, which have deep taproots or extensive underground systems. While the foliage may die back, the root will likely survive and regrow, necessitating repeated applications.
Safety and Environmental Impact of Using Ammonia
Using ammonia as a weed killer introduces significant human safety hazards. Ammonia fumes are corrosive to the respiratory tract and eyes, so eye protection, gloves, and working in a well-ventilated area are mandatory precautions. Under no circumstances should ammonia be mixed with any other cleaning product, particularly chlorine bleach, as this combination creates highly toxic chloramine gases.
The environmental risks are substantial, extending beyond the immediate contact kill of desirable plants. When the ammonia solution soaks into the soil, the high concentration of nitrogen can cause nitrogen burn, damaging the roots of nearby turf or ornamental plants. An excessive localized application acts as an extreme overdose, shifting the nitrogen from a nutrient to a toxin.
Furthermore, the high alkalinity of the ammonia solution can temporarily spike the soil’s pH, which may harm beneficial soil microorganisms essential for nutrient cycling. If the solution runs off into nearby ponds or streams, it poses a direct threat to aquatic life, which is highly sensitive to ammonia. The treated area also remains toxic to pets that may ingest the treated plant material or lick the area before the chemical has fully dissipated.